Cold rolling method, cold rolling equipment, and cold-rolled steel sheet manufacturing method

ABSTRACT

A cold rolling method, cold rolling equipment, and a cold-rolled steel sheet manufacturing method capable of preventing sheet breakage by sufficiently suppressing occurrence of an edge crack of a material to be rolled during cold rolling. In the cold rolling method, a rolling mill including a plurality of stands to cold-rolls a material to be rolled. An N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter. The N-th stand rolls the material to be rolled with a linear load of 0.8 t/mm or more.

TECHNICAL FIELD

The present invention relates to a cold rolling method for cold-rolling a material to be rolled, cold rolling equipment, and a cold-rolled steel sheet manufacturing method.

BACKGROUND ART

In a cold-rolled steel sheet manufacturing line, sheet breakage may occur due to various factors during cold rolling of a material to be rolled. One of the reasons for the sheet breakage is occurrence of an edge crack based on a crack which occurred due to a tension applied to an edge portion increased due to a defective sheet thickness distribution (for example, edge drop) in a sheet width direction of the material to be rolled, and this may lead to the sheet breakage. In order to reduce such sheet breakage, it is important to suppress the generation of cracks and an increase of the generated cracks in their size. The edge drop refers to a phenomenon of rapid sheet thickness reduction particularly at both end portions in a sheet width direction in a sheet thickness deviation in the sheet width direction that occurs on a sheet material during the rolling.

In the related art, a method for controlling a sheet thickness of a sheet material in a width direction disclosed in PTL 1 has been proposed as, for example, a method for reducing the amount of the edge drop and improving the sheet thickness distribution in the sheet width direction.

In the method for controlling the sheet thickness in the width direction of a sheet material disclosed in PTL 1, the sheet thickness of the sheet material in the width direction is controlled by a rolling mill including a mechanism of shifting and a mechanism of crossing a work roll having a tapered roll end portion in a plurality of stands.

According to the method for controlling the sheet thickness of the sheet material in the width direction disclosed in PTL 1, it is possible to accurately perform the control in a divided manner using the plurality of stands. Accordingly, it is possible to obtain an excellent sheet thickness distribution over the entire sheet width from a sheet thickness deviation that is gently generated from a sheet width center toward a sheet end side to a sheet thickness deviation sharply generated on a sheet width end portion (edge drop).

CITATION LIST Patent Literature

PTL 1: JP H10-29010 A

SUMMARY OF INVENTION Technical problem

However, the method for controlling the sheet thickness of the sheet material in the width direction disclosed in PTL 1 has the following problems.

That is, in the method for controlling the sheet thickness of the sheet material in the width direction disclosed in PTL 1, the excellent sheet thickness distribution is obtained over the entire sheet width, but according to a test of the inventors, it was found that the occurrence of edge cracks of a material to be rolled during cold rolling cannot be sufficiently suppressed and sheet breakage during the cold rolling cannot be sufficiently suppressed.

Therefore, the present invention has been made to solve the problems of the related art, and an object thereof is to provide a cold rolling method capable of preventing sheet breakage by sufficiently suppressing occurrence of an edge crack of a material to be rolled during cold rolling, cold rolling equipment, and a cold-rolled steel sheet manufacturing method.

Solution to Problem

In order to achieve the aforementioned object, there is provided a cold rolling method according to an aspect of the present invention for cold-rolling a material to be rolled by a rolling mill including a plurality of stands, in which an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and the N-th stand rolls the material to be rolled with a linear load of 0.8 t/mm or more.

In addition, there is provided a cold rolling method according to another aspect of the present invention for cold-rolling a material to be rolled by a rolling mill including a plurality of stands, in which an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction and an (N+1)-th stand arranged in an (N+1)-th position on a downstream side of the N-th stand among the plurality of stands, include a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and each of the N-th stand and the (N+1)-th stand rolls the material to be rolled with a linear load of 1.7 t/mm or more and a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of each of the N-th stand and the (N+1)-th stand is set as −50 mm to −5 mm.

In addition, there is provided a cold rolling equipment according to still another aspect of the present invention having a rolling mill including a plurality of stands for rolling a material to be rolled, in which an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and a linear load of the N-th stand is set as 0.8 t/mm or more.

In addition, there is provided a cold rolling equipment according to still another aspect of the present invention including a rolling mill having a plurality of stands for rolling a material to be rolled, in which each of an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction and an (N+1)-th stand arranged in an (N+1)-th position on a downstream side of the N-th stand among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and a linear load of each of the N-th stand and the (N+1)-th stand is set as 1.7 t/mm or more, and a tapered rolling portion width WRδ, which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of each of the N-th stand and the (N+1)-th stand is set as −50 mm to −5 mm.

In addition, there is provided a cold-rolled steel sheet manufacturing method according to still another aspect of the present invention including cold-rolling a steel sheet to manufacture a cold-rolled steel sheet by the cold rolling method described above.

Advantageous Effects of Invention

According to the cold rolling method, the cold rolling equipment, and the cold-rolled steel sheet manufacturing method according to the present invention, it is possible to prevent sheet breakage by sufficiently suppressing occurrence of an edge crack of a material to be rolled during cold rolling.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram of cold rolling equipment according to a first embodiment of the present invention;

FIG. 2 is a view for explaining a tapered work roll in the cold rolling equipment illustrated in FIG. 1 ;

FIG. 3 is a diagram for explaining an edge drop ratio;

FIG. 4 is a diagram for explaining edge-up;

FIG. 5 is a graph illustrating an edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.4 t/mm, in a case where a work roll of a second stand is set as a tapered work roll and work rolls of a first stand, a third stand, and a fourth stand are set as flat work rolls;

FIG. 6 is a graph illustrating sheet crowns of the second stand and the third stand on an outer side, in a case where the work rolls of the second stand and the third stand are set as the flat work rolls;

FIG. 7 is a graph illustrating tensions of the second stand and the third stand on an outer side, in a case where the work rolls of the second stand and the third stand are set as the flat work rolls;

FIG. 8 is a graph illustrating sheet crowns of the second stand and the third stand on an outer side, in a case where the work roll of the second stand is set as the tapered work roll and the work roll of the third stand is set as the flat work roll;

FIG. 9 is a graph illustrating tensions of the second stand and the third stand on an outer side, in a case where the work roll of the second stand is set as the tapered work roll and the work roll of the third stand is set as the flat work roll;

FIG. 10 is a graph illustrating comparison between a breakage occurrence ratio in a case where the work roll of the second stand is set as the tapered work roll and the work roll of the third stand is set as the flat work roll, and a breakage occurrence ratio in a case where the work rolls of the second stand and the third stand are set as the flat work rolls;

FIG. 11 is a graph illustrating comparison of the edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.25 t/mm by changing a tapered rolling portion width WRδ of the tapered work roll of the second stand to +20 mm, +50 mm, −5 mm, and +60 mm, in a case where the work roll of the second stand is set as the tapered work roll and the work rolls of the first stand, the third stand, and the fourth stand are set as the flat work rolls;

FIG. 12 is a graph illustrating comparison of the edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.35 t/mm by changing a tapered rolling portion width WRδ of the tapered work roll of the second stand to +20 mm, +50 mm, −5 mm, and +60 mm, in a case where the work roll of the second stand is set as the tapered work roll and the work rolls of the first stand, the third stand, and the fourth stand are set as the flat work rolls;

FIG. 13 is a graph illustrating comparison of the edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.45 t/mm by changing a tapered rolling portion width WRδ of the tapered work roll of the second stand to −20 mm, −50 mm, +5 mm, and −60 mm, in a case where the work roll of the second stand is set as the tapered work roll and the work rolls of the first stand, the third stand, and the fourth stand are set as the flat work rolls;

FIG. 14 is a graph illustrating comparison of the edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.60 t/mm by changing a tapered rolling portion width WRδ of the tapered work roll of the second stand to −20 mm, −50 mm, +5 mm, and −60 mm, in a case where the work roll of the second stand is set as the tapered work roll and the work rolls of the first stand, the third stand, and the fourth stand are set as the flat work rolls;

FIG. 15 is a schematic configuration diagram of cold rolling equipment according to a second embodiment of the present invention; and

FIG. 16 is a graph illustrating an edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.7 t/m, in a case where the work rolls of all the first stand to the fourth stand are set as the flat work rolls, in a case where only the work roll of the second stand is set as the tapered work roll and the work rolls of the other stands are set as the flat work rolls, and in a case where the work rolls of the second stand and the third stand are set as the tapered work rolls and the work roll of the other stands are set as the flat work rolls.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments illustrated below are examples of a device or a method for implementing technical idea of the present invention and the technical idea of the present invention does not intend to specify materials, shapes, structures, arrangement, and the like of the constituent components to the following embodiments. Also, the drawings are schematically illustrated. Therefore, it should be noted that a relationship, a ratio, and the like between a thickness and a planar dimension are different from the actual ones, and the drawings include portions where the relationship or ratio thereof are different from each other.

First Embodiment

FIG. 1 illustrates a schematic configuration of cold rolling equipment according to a first embodiment of the present invention.

Cold rolling equipment 1 illustrated in FIG. 1 includes a rolling mill (tandem rolling mill) 2 including a plurality of (four in this embodiment) stands 3 ₁ to 3 ₄. The rolling mill 2 cold-rolls a steel sheet S (see FIG. 2 ) as a material to be rolled. A steel sheet S is rolled by the rolling mill 2 while being transferred from an upstream side to a downstream side of the cold rolling equipment 1. The stands are configured with a first stand 3 ₁ arranged in a first position from an upstream side of the steel sheet S in a transfer direction, a second stand 3 ₂ arranged on a downstream side of the first stand 3 ₁, a third stand 3 ₃ arranged on a downstream side of the second stand 3 ₂, and a fourth stand 3 ₄ arranged on a downstream side of the third stand 3 ₃. The number of stands is not limited to four and may be any number greater than 1. The subscript of each stand indicates the arrangement order from the upstream side of the steel sheet S in the transfer direction, and a stand arranged in an N-th position (N is a natural number equal to or greater than 2) from the upstream side of the steel sheet S in the transfer direction is referred to as an N-th stand 3 _(N), and a stand arranged in an (N+1)-th position is referred to as an (N+1)-th stand 3_(N+1), . . . . In FIG. 1 , for reference, the N-th stand 3_(N) is parenthesized and written together with the second stand 3 ₂, and the (N+1)-th stand 3_(N+1) is parenthesized and written together with the third stand 3 ₃.

Each of the first stand 3 ₁ to the fourth stand 3 ₄ includes a pair of upper and lower work rolls 4 a for rolling the steel sheet S as a material to be rolled, a pair of upper and lower back-up rolls 4 b for supporting the work rolls 4 a, and a pair of upper and lower intermediate rolls 4 c arranged between each work roll 4 a and each back-up roll 4 b.

In addition, each work roll 4 a of the second stand 3 ₂ is configured as a tapered work roll 4 a 1 (hatched in FIG. 1 ), and each work roll 4 a of the first stand 3 ₁, the third stand 3 ₃, and the fourth stand 3 ₄ other than the second stand 3 ₂ is configured with a flat work roll 4 a 2 (not hatched in FIG. 1 ). The configurations of the tapered work roll 4 a 1 and the flat work roll 4 a 2 will be described later.

The reason why each work roll 4 a of the second stand 3 ₂ is configured with the tapered work roll 4 a 1 as described above will be described below.

As illustrated in FIG. 1 , the present inventors had conducted studies about a stand where an edge crack occurs when an electromagnetic steel sheet, as a material to be rolled, is cold-rolled by the rolling mill (tandem rolling mill) 2 including four stands which are first stand 3 ₁ to the fourth stand 3 ₄. In the rolling mill 2 used in this test, among the four stands which are the first stand 3 ₁ to the fourth stand 3 ₄, the work rolls 4 a of the first stand 3 ₁ and the fourth stand 3 ₄ were set as the tapered work rolls 4 a 1, and the work rolls 4 a of the second stand 3 ₂ and the third stand 3 ₃ were set as the flat work rolls 4 a 2.

As a result of investigating a starting point of a crack by rolling the electromagnetic steel sheet with the rolling mill 2, it was confirmed that the edge crack occurred on an outer side of the third stand 3 ₃. In order to identify the factor, an edge drop ratio of each of the first stand 3 ₁ to the fourth stand 3 ₄ was measured.

Here, the edge drop refers to a phenomenon of rapid sheet thickness reduction particularly at both end portions in a sheet width direction in a sheet thickness deviation in the sheet width direction that occurs on the steel sheet S during the rolling. The edge drop ratio is represented by the following equation, in a case where a sheet thickness at 5 mm from both end surfaces of the steel sheet S in the sheet width direction is defined as E5 and a sheet thickness at 20 mm from the both end surfaces is defined as E20, as illustrated in FIG. 3 .

Edge drop ratio Ed=(E5−E20)/E20

An edge-up known as a sheet thickness deviation in contrast to the edge drop refers to a phenomenon of a rapid sheet thickness increase particularly at both end portions in the sheet width direction in the sheet thickness deviation in the sheet width direction that occurs on the steel sheet S during the rolling. An edge-up amount is represented by the following equation, in a case where a sheet thickness at 5 mm from both end surfaces of the steel sheet S in the sheet width direction is defined as E5 and a sheet thickness at 20 mm from the both end surfaces is defined as E20, as illustrated in FIG. 4 .

Edge-up amount Eu=E5−E20

In FIGS. 3 and 4 , a reference numeral CL represents a center line of the steel sheet S in a width direction.

As a result of measuring the edge drop ratio of each of the first stand 3 ₁ to the fourth stand 3 ₄, it was found that the edge drop ratio was greatly increased and exceeded a suitable range on the third stand 3 ₃, and then the edge drop ratio was decreased on the fourth stand 3 ₄. Here, an increase in the edge drop ratio means that a numerical value of the edge drop ratio increases on a negative side, and a decrease in the edge drop ratio means that a numerical value of the edge drop ratio decreases on a negative side.

In other words, it was found that, in a case where a difference between the edge drop ratio on the fourth stand 3 ₄ and the edge drop ratio on the third stand 3 ₃ is large (the difference is 0.02 or more on a+side), the sheet breakage easily occurs.

It was assumed that this was because an excessive tension was applied to both edge portions of an electromagnetic steel sheet in a sheet width direction in the third stand 3 ₃ and the edge crack occurred on the outer side of the third stand 3 ₃ due to a crack or the like on an edge portion occurred in the second stand 3 ₂ on the upstream side as a starting point.

For this reason, the present inventors make a uniform sheet thickness distribution of the steel sheet by setting the work roll 4 a of the second stand 3 ₂ on the upstream side as the tapered work roll 4 a 1, and set the work roll 4 a of the third stand 3 ₃ on the downstream side as the flat work roll 4 a 2. Accordingly, the present inventors acquired findings in that the edge drop ratio on the first stand 3 ₁, the second stand 3 ₂, the third stand 3 ₃, and the fourth stand 3 ₄ can be set in the suitable range, the difference between the edge drop ratio on the fourth stand 3 ₄ and the edge drop ratio on the third stand 3 ₃ is decreased, and the tension applied to the edge portion of the third stand 3 ₃ on the downstream side is decreased, thereby preventing the edge crack of the steel sheet.

For this reason, each work roll 4 a of the second stand 3 ₂ was configured with the tapered work roll 4 a 1. Accordingly, it was found that, by performing the rolling with the tapered work roll 4 a 1 in the second stand 3 ₂, the occurrence of the excessive edge drop was suppressed, and even when the rolling is performed with the flat work roll in the subsequent third stand 3 ₃, the tension applied to the edge portion was decreased, thereby suppressing the occurrence of the crack. As a result of cold-rolling the electromagnetic steel sheet as the material to be rolled by the rolling mill 2 illustrated in FIG. 1 , the breakage caused by the crack of the edge portion of the electromagnetic steel sheet was reduced.

FIG. 5 illustrates an edge drop ratio on each of the first stand 3 ₁ to the fourth stand 3 ₄ when each of the first stand 3 ₁ to the fourth stand 3 ₄ rolls the steel sheet S with a linear load of 1.4 t/mm, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 (although will be described later, a tapered rolling portion width WRδ of the second stand 3 ₂ is −50 mm to −5 mm) and the work rolls 4 a of the other stands 3 ₁, 3 ₃, and 3 ₄ are set as the flat work rolls 4 a 2. As illustrated in FIG. 5 , it was found that, when the tapered work roll 4 a 1 was applied to the second stand 3 ₂, the edge drop ratio of the first stand 3 ₁ to the fourth stand 3 ₄ was in the suitable ranges, without significant increase, and the difference between the edge drop ratio on the fourth stand 3 ₄ and the edge drop ratio on the third stand 3 ₃ was less than 0.02.

FIG. 6 illustrates a sheet crown on an outer side of each of the second stand 3 ₂ and the third stand 3 ₃ in a case where the work rolls 4 a of the second stand 3 ₂ and the third stand 3 ₃ were set as the flat work rolls 4 a 2. In addition, FIG. 7 illustrates a tension on an outer side of each of the second stand 3 ₂ and the third stand 3 ₃ in a case where the work rolls 4 a of the second stand 3 ₂ and the third stand 3 ₃ were set as the flat work rolls 4 a 2.

As illustrated in FIG. 6 , in a case where the flat work roll 4 a 2 is applied to the second stand 3 ₂ and the third stand 3 ₃, the excessive edge drop occurs in the second stand 3 ₂. Meanwhile, the edge drop ratio greatly increases from the second stand 3 ₂ to the third stand 3 ₃, and excessive tension is applied to the third stand 3 ₃, as illustrated in FIG. 7 .

In addition, FIG. 8 illustrates a sheet crown on the outer side of each of the second stand 3 ₂ and the third stand 3 ₃, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 and the work roll 4 a of the third stand 3 ₃ is set as the flat work roll 4 a 2. Further, FIG. 9 illustrates a tension on the outer side of each of the second stand 3 ₂ and the third stand 3 ₃, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 and the work roll 4 a of the third stand 3 ₃ is set as the flat work roll 4 a 2.

As illustrated in FIG. 8 , it is found that, when the tapered work roll 4 a 1 is applied to the second stand 3 ₂ and the flat work roll 4 a 2 is applied to the third stand 3 ₃, the excessive edge drop is suppressed on the second stand 3 ₂, the edge drop is suitably promoted on the third stand 3 ₃, and accordingly, an edge tension is suppressed from becoming an excessive tension as illustrated in FIG. 9 .

When each breakage occurrence ratio is compared, the breakage occurrence ratio can be decreased by configuring the work roll 4 a of the second stand 3 ₂ with the tapered work roll 4 a 1 as illustrated in FIG. 10 , compared to a case where the work roll 4 a of the second stand 3 ₂ is configured with the flat work roll 4 a 2.

From the above, by configuring the work roll 4 a of the second stand 3 ₂ with the tapered work roll 4 a 1, it is possible to reinforce the findings in that the occurrence of the excessive edge drop on the second stand 3 ₂ on the upstream side can be suppressed and the tension applied to the edge portion of the steel sheet S on the third stand 3 ₃ on the downstream side can be decreased to suppress the occurrence of the edge crack.

The above result is obtained in a case where the second stand 3 ₂ having the tapered work rolls 4 a 1 rolls the steel sheet S with a linear load (rolling load/sheet width) of 1.4 t/mm.

As a result of further tests, as illustrated in Tables 1, FIG. 11 , Table 2, and FIG. 12 , it was found that, in a case where the linear load of the second stand 3 ₂ having the tapered work rolls 4 a 1 was set as less than 1.4 t/mm, the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is preferably ±0 mm to +50 mm. Although the tapered rolling portion width WRδ will be described in detail later, when the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as a value on a negative side, that is, by shifting the tapered work roll 4 a 1 of the second stand 3 ₂ to a negative direction, the steel sheet S is reversely edged up on the shifted second stand 3 ₂, thereby causing the occurrence of the edge crack or the breakage. Here, in a case of edge crack, the steel sheet S cannot pass through a next step (annealing step), and since there is a high possibility that the steel sheet S is broken due to the annealing, it is preferable to roll the steel sheet S by shifting the tapered work roll 4 a 1 of the second stand 3 ₂ to the positive direction. Even in a case where the tapered work roll 4 a 1 of the second stand 3 ₂ is shifted in the positive direction, when the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is larger than +50 mm, there is a risk that the steel sheet S is edged up on the next third stand 3 ₃ after the shifted second stand 3 ₂ to cause the occurrence of the edge crack or the breakage. Accordingly, it is preferable that an upper limit of the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is set as +50 mm.

TABLE 1 Linear load 1.25 (t/mm) WRδ Edge drop ratio (−) (mm) Base sheet First stand Second stand Third stand Fourth stand +20 −0.001 −0.013 −0.043 −0.063 −0.083 +50 −0.001 −0.013 −0.063 −0.083 −0.088 −5 −0.001 −0.013 0.007 −0.013 −0.053 +60 −0.001 −0.013 −0.083 −0.053 −0.063

TABLE 2 Linear load 1.35 (t/mm) WRδ Edge drop ratio (−) (mm) Base sheet First stand Second stand Third stand Fourth stand +20 −0.001 −0.013 −0.043 −0.073 −0.093 +50 −0.001 −0.013 −0.073 −0.103 −0.108 −5 −0.001 −0.013 −0.003 −0.033 −0.073 +60 −0.001 −0.013 −0.093 −0.073 −0.083

Table 1 and FIG. 11 illustrate comparison of the edge drop ratio on each stand when each stand rolls the steel sheet with a linear load of 1.25 t/mm by changing the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand to +20 mm, +50 mm, −5 mm, and +60 mm, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 and the work rolls 4 a of the first stand 3 ₁, the third stand 3 ₃, and the fourth stand 3 ₄ are set as the flat work rolls 4 a 2.

As illustrated in Table 1 and FIG. 11 , it is found that, when the second stand 3 ₂ having the tapered work roll 4 a 1 rolls the steel sheet S with a linear load of 1.25 t/mm, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −5 mm beyond the range, the second stand 3 ₂ shifted in the negative direction is edged up, and in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as +60 mm beyond the range, the next second stand 3 ₃ after the second stand 3 ₂ shifted in the positive direction is edged up. It is found that, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as +20 mm and +50 mm in the range, the edge drop of the second stand 3 ₂, the third stand 3 ₃, the fourth stand 3 ₄ is suitably promoted.

In addition, Table 2 and FIG. 12 illustrate comparison of the edge drop ratio on each stand when each stand rolls the steel sheet with a linear load of 1.35 t/mm by changing the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand to +20 mm, +50 mm, −5 mm, and +60 mm, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 and the work rolls 4 a of the first stand 3 ₁, the third stand 3 ₃, and the fourth stand 3 ₄ are set as the flat work rolls 4 a 2.

As illustrated in Table 2 and FIG. 12 , it is found that, when the second stand 3 ₂ having the tapered work roll 4 a 1 rolls the steel sheet S with a linear load of 1.35 t/mm, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −5 mm beyond the range, the second stand 3 ₂ shifted in the negative direction is edged up, and in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as +60 mm beyond the range, the next second stand 3 ₃ after the second stand 3 ₂ shifted in the positive direction is edged up. It is found that, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as +20 mm and +50 mm in the range, the edge drop of the second stand 3 ₂, the third stand 3 ₃, and the fourth stand 3 ₄ is suitably performed.

A lower limit value of the linear load at which the edge crack of the steel sheet S occurs is 0.8 t/mm, unless the second stand 3 ₂ is shifted in the positive direction. In other words, in a case where the steel sheet S is rolled with a linear load of less than 0.8 t/mm, the edge crack of the steel sheet S does not occur regardless of the shift direction of the second stand 3 ₂.

In addition, as a result of further tests, as illustrated in Tables 3, FIG. 13 , Table 4, and FIG. 14 , it was found that, in a case where the linear load of the second stand 3 ₂ having the tapered work rolls 4 a 1 was set as 1.4 t/mm or more, the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is preferably −50 mm to −5 mm. When the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as a value on a positive side, that is, by shifting the tapered work roll 4 a 1 of the second stand 3 ₂ to a positive direction, the steel sheet S is edged up on the next stand (the third stand 3 ₃) or the stand after the next stand (the fourth stand 3 ₄) after the shifted second stand 3 ₂, thereby causing the occurrence of the edge crack or the breakage. The expression that “it was found that, in a case where a difference between the edge drop ratio on the fourth stand 3 ₄ and the edge drop ratio on the third stand 3 ₃ is large, the sheet breakage easily occurs” has the same meaning as the expression “the steel sheet S is edged up on the next stand (the fourth stand 3 ₄) after the shifted second stand 3 ₂, thereby causing the occurrence of the edge crack or the breakage”. Here, in a case of the edge crack, the steel sheet S cannot pass through the next step (annealing step) and a possibility that the steel sheet S cracks due to the annealing is high. In addition, when the steel sheet S is rolled by shifting the tapered work roll 4 a 1 of the second stand 3 ₂ in the positive direction, the edge drop is excessively produced on the shifted second stand 3 ₂ (excessive edge drop occurs), and accordingly, a pressure of the edge of the steel sheet S cannot be decreased on the next stand (third stand 3 ₃) and the edge crack may occur due to an increase in tension. Therefore, it is preferable to roll the steel sheet S by shifting the second stand 3 ₂ in the negative direction. Even in a case where the second stand 3 ₂ is shifted in the negative direction, when the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is larger than −50 mm, there is a risk that the steel sheet S is edged up on the shifted second stand 3 ₂ to cause the occurrence of the edge crack or the breakage. Accordingly, it is preferable that a lower limit of the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is set as −50 mm.

TABLE 3 Linear load 1.45 (t/mm) WRδ Edge drop ratio (−) (mm) Base sheet First stand Second stand Third stand Fourth stand +20 −0.001 −0.013 −0.033 −0.053 −0.073 +50 −0.001 −0.013 −0.023 −0.043 −0.048 −5 −0.001 −0.013 −0.113 −0.063 −0.103 +60 −0.001 −0.013 0.007 −0.013 −0.023

TABLE 4 Linear load 1.60 (t/mm) WRδ Edge drop ratio (−) (mm) Base sheet First stand Second stand Third stand Fourth stand +20 −0.001 −0.013 −0.043 −0.073 −0.093 +50 −0.001 −0.013 −0.033 −0.043 −0.048 −5 −0.001 −0.013 −0.123 −0.053 −0.093 +60 −0.001 −0.013 0.047 −0.017 −0.007

Table 3 and FIG. 13 illustrate comparison of the edge drop ratio on each stand when each stand rolls the steel sheet with a linear load of 1. 45 t/mm by changing the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand to −20 mm, −50 mm, +5 mm, and −60 mm, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 and the work rolls 4 a of the first stand 3 ₁, the third stand 3 ₃, and the fourth stand 3 ₄ are set as the flat work rolls 4 a 2.

As illustrated in Table 3 and FIG. 13 , it is found that, when the second stand 3 ₂ having the tapered work roll 4 a 1 rolls the steel sheet S with a linear load of 1.45 t/mm, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the second stand 3 ₂ as +5 mm beyond the range, the excessive edge drop occurs on the second stand 3 ₂ shifted in the positive direction and the next third stand 3 ₂ is edged up, and in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as −60 mm beyond the range, the second stand 3 ₂ shifted in the negative direction is edged up. It is found that, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as −20 mm and −50 mm in the range, the edge drop of the second stand 3 ₂, the third stand 3 ₃, the fourth stand 3 ₄ is suitably promoted.

In addition, Table 4 and FIG. 14 illustrate comparison of the edge drop ratio on each stand when each stand rolls the steel sheet with a linear load of 1.60 t/mm by changing the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand to −20 mm, −50 mm, +5 mm, and −60 mm, in a case where the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1 and the work rolls 4 a of the first stand 3 ₁, the third stand 3 ₃, and the fourth stand 3 ₄ are set as the flat work rolls 4 a 2.

As illustrated in Table 4 and FIG. 14 , it is found that, when the second stand 3 ₂ having the tapered work roll 4 a 1 rolls the steel sheet S with a linear load of 1.60 t/mm, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as +5 mm beyond the range, the excessive edge drop occurs on the second stand 3 ₂ shifted in the positive direction and the next third stand 3 ₂ is edged up, and in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as −60 mm beyond the range, the second stand 3 ₂ shifted in the negative direction is edged up. It is found that, in a case where the steel sheet S is rolled by setting the tapered rolling portion width WRδ as −20 mm and −50 mm in the range, the edge drop of the second stand 3 ₂, the third stand 3 ₃, the fourth stand 3 ₄ is suitably promoted.

From the above, the work roll 4 a of the second stand 3 ₂ is configured with the tapered work roll 4 a 1, and the second stand 3 ₂ having the tapered work roll 4 a 1 rolls the steel sheet S with a linear load of 0.8 t/mm or more. When the second stand 3 ₂ rolls the steel sheet S with a linear load of less than 1.4 t/mm, the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll. 4 a 1 of the second stand 3 ₂ as ±0 mm to +50 mm. In addition, when the second stand 3 ₂ rolls the steel sheet S with a linear load of 1.4 t/mm or more, the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −50 mm to −5 mm. It was found that by this, both edge drop and edge-up can be appropriately controlled, and the occurrence of edge crack can be suppressed.

The reason for determining whether to shift the tapered work roll 4 a 1 of the second stand 3 ₂ in the positive direction or shift the tapered work roll 4 a 1 in the negative direction based on the linear load, 1.4 t/mm, of the second stand 3 ₂ having the tapered work roll 4 a 1 is as described below. That is, in a case where the linear load of the second stand 3 ₂ having the tapered work roll 4 a 1 is set as 1.4 t/mm or more, the material to be rolled is a high-load material, and the tapered work roll 4 a 1 easily bend. Accordingly, by shifting the tapered work roll 4 a 1 in the negative direction, the excessive edge drop is suppressed. On the other hand, in a case where the linear load of the second stand 3 ₂ having the tapered work roll 4 a 1 is set as less than 1.4 t/mm, the material to be rolled is a low-load material, and the tapered work roll 4 a 1 is less likely to bend and the edge-up easily occurs. Accordingly, by shifting the tapered work roll 4 a 1 in the positive direction, it is held on the edge drop side.

Next, the configuration of the tapered work roll 4 a 1 will be described with reference to FIG. 2 . In the tapered work roll 4 a 1, a taper 4 ab which is tapered is formed on an end portion of a roll 4 aa having a uniform diameter in a body length direction. The work roll 4 a, which is the tapered work roll 4 a 1, is configured to be shifted in an axial direction (a roll body length direction, a sheet width direction of the steel sheet S).

The tapered rolling portion width WRδ of the tapered work roll 4 a 1 is a length of the taper 4 ab facing the steel sheet S, and is a length from a taper start end 4 ac to a width direction end surface of the steel sheet S in FIG. 2 . As described above, when the second stand 3 ₂ rolls the steel sheet S with a linear load of less than 1.4 t/mm, the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as ±0 mm to +50 mm. That is, it is preferable to roll the steel sheet S by shifting the tapered work roll 4 a 1 of the second stand 3 ₂ in the positive direction.

In addition, as described above, when the second stand 3 ₂ rolls the steel sheet S with a linear load of 1.4 t/mm or more, the steel sheet S is rolled by setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −50 mm to −5 mm. That is, it is preferable to roll the steel sheet S by shifting the tapered work roll 4 a 1 of the second stand 3 ₂ in the negative direction.

In a case where the width direction end surface of the steel sheet S protrudes from the taper start end 4 ac, the tapered rolling portion width WRδ is a negative value, and in a case where the width direction end surface of the steel sheet S is recessed from the taper start end 4 ac, the tapered rolling portion width WRδ is a positive value. A case where the tapered rolling portion width WRδ is a positive value is the same operation as a case where the flat work roll 4 a 2 is used.

In addition, an inclination of the taper 4 ab is represented as L/H, in a case where a length from the taper start end 4 ac to a taper final end surface (axial direction end surface of the tapered work roll 4 a 1) is defined as L and a difference in height between an outer peripheral surface of the roll 4 aa and an outer periphery of the taper final end surface is defined as H. The inclination L/H of the taper 4 ab is preferably 1/800 to 1/400. In a case where the inclination L/H of the taper 4 ab is smaller than 1/800, there is a problem that the edge drop cannot be suppressed. On the other hand, in a case where the inclination L/H of the taper 4 ab is larger than 1/400, there is a problem that the edge-up excessively occurs.

In addition, in the description of the configuration of the flat work roll 4 a 2, the flat work roll 4 a 2 is configured with a roll having a uniform diameter in the body length direction. The work roll 4 a, which is the flat work roll 4 a 2, is configured to be shifted in an axial direction (a roll body length direction, a sheet width direction of the steel sheet S).

As described above, in the cold rolling equipment 1 according to the first embodiment, among the four stands of the first stands 3 ₁ to the fourth stands 3 ₄, the second stand 3 ₂ arranged in the second position from the upstream side of the steel sheet S, as the material to be rolled, in the transfer direction includes the tapered work roll 4 a 1 having the taper 4 ab formed on the end portion of the roll 4 aa having a uniform diameter. A linear load of the second stand 3 ₂ is set as 0.8 t/mm or more. When the linear load of the second stand 3 ₂ is set as less than 1.4 t/mm, the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is ±0 mm and +50 mm. In addition, when the linear load of the second stand 3 ₂ is set as 1.4 t/mm or more, the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is set as −50 mm to −5 mm.

Therefore, the edge drop ratio on the first stand 3 ₁ to the fourth stand 3 ₄ is kept within the appropriate range, and the occurrence of edge crack on the steel sheet S as the material to be rolled during the cold rolling is sufficiently suppressed, thereby suppressing the sheet breakage.

In addition, in the cold rolling equipment 1 according to the first embodiment, among the four stands of first stands 3 ₁ to the fourth stand 3 ₄, the third stand 3 ₃ arranged in the third position on the downstream side of the second stand 3 ₂ includes the flat work roll 4 a 2 having a uniform diameter of a roll.

Accordingly, the effect of promoting the edge drop is exhibited.

In addition, in the cold rolling equipment 1 according to the first embodiment, among the four stands of first stands 3 ₁ to the fourth stands 3 ₄, the first stand 3 ₁ arranged on the uppermost stream side includes the flat work roll 4 a 2, but may include the tapered work roll 4 a 1.

Accordingly, the effect of suppressing the edge drop is exhibited.

In addition, in a cold rolling method according to the first embodiment, the steel sheet S as the material to be rolled is cold-rolled by the rolling mill 2 in the cold rolling equipment 1 illustrated in FIG. 1 . In this case, among the four stands of the first stands 3 ₁ to the fourth stands 3 ₄, the second stand 3 ₂ arranged in the second position from the upstream side of the steel sheet S in the transfer direction includes the tapered work roll 4 a 1 having the taper 4 ab formed on the end portion of the roll 4 aa having a uniform diameter, and the second stand 3 ₂ rolls the steel sheet S with a linear load of 0.8 t/mm or more.

When the second stand 3 ₂ rolls the steel sheet S with a linear load of less than 1.4 t/mm, the steel sheet S is rolled by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of the second stand 3 ₂ as ±0 mm to +50 mm.

Meanwhile, when the second stand 3 ₂ rolls the steel sheet S with a linear load of 1.4 t/mm or more, the steel sheet S is rolled by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of the second stand 3 ₂ as −50 mm to −5 mm.

Therefore, the edge drop ratio on the first stand 3 ₁ to the fourth stand 3 ₄ is kept within the appropriate range, and the occurrence of edge crack on the steel sheet S as the material to be rolled during the cold rolling is sufficiently suppressed, thereby suppressing the sheet breakage.

In the cold-rolled steel sheet manufacturing method according to the first embodiment, the steel sheet S is cold-rolled by the cold rolling method according to the first embodiment to manufacture the cold-rolled steel sheet.

Second Embodiment

Next, cold rolling equipment according to a second embodiment of the present invention will be described with reference to FIG. 15 . FIG. 15 illustrates a schematic configuration of cold rolling equipment according to a second embodiment of the present invention. In FIG. 15 , the same reference numerals are used for the same members as the members of FIG. 1 and the description thereof may be omitted. In FIG. 15 , for reference, the N-th stand 3 _(N) is parenthesized and written together with the second stand 3 ₂, the (N+1)-th stand 3_(N+1) is parenthesized and written together with the third stand 3₃, and the (N+2)-th stand 3_(N+2) is parenthesized and written together with the fourth stand 3 ₄.

Unlike the cold rolling equipment 1 illustrated in FIG. 1 , in the cold rolling equipment 1 illustrated in FIG. 15 , each work roll 4 a of the second stand 3 ₂ is not only configured with the tapered work roll 4 a 1 (hatched in FIG. 15 ), but also each work roll 4 a of the third stand 3 ₃ is configured with the tapered work roll 4 a 1. Each work roll 4 a of the first stand 3 ₁ and the fourth stand 3 ₄ is configured with the flat work roll 4 a 2.

The reason why each work roll 4 a of the second stand 3 ₂ and each work roll 4 a of the third stand 3 ₃ is configured with the tapered work roll 4 a 1 as described above will be described below.

FIG. 16 illustrates an edge drop ratio on each stand when each stand rolls a steel sheet with a linear load of 1.7 t/m, in a case where the work rolls of all the first stand to the fourth stand are set as the flat work rolls, in a case where only the work roll of the second stand is set as the tapered work roll (the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ is set as −50 mm to −5 mm) and the work rolls of the other stands are set as the flat work rolls, and in a case where the work rolls of the second stand and the third stand are set as the tapered work rolls (each of the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ and the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the third stand 3 ₃ is set as −50 mm to −5 mm) and the work roll of the other stands are set as the flat work rolls.

As illustrated in FIG. 16 , in a case where the second stand 3 ₂ having the tapered work rolls 4 a 1 (the tapered rolling portion width WRδ of the second stand 3 ₂ is set as −50 mm to −5 mm) rolls an electromagnetic steel sheet with a linear load of 1.7 t/mm, the edge drop ratio of the first stand 3 ₁ to the fourth stand 3 ₄ is set in the suitable range, without a great increase. The same effect is also exhibited in a case where the second stand 3 ₂ having the tapered work rolls 4 a 1 (the tapered rolling portion width WRδ of the second stand 3 ₂ is set as −50 mm to −5 mm) rolls the electromagnetic steel sheet with a large linear load of greater than 1.7 t/mm.

On the other hand, as illustrated in FIG. 16 , in a case where the second stand 3 ₂ and the third stand 3 ₃ having two continuous tapered work rolls 4 a 1 (each of the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ and the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the third stand 3 ₃ is set as −50 mm to −5 mm) rolls the electromagnetic steel sheet with a linear load of 1.7 t/mm, an increase ratio from the edge drop ratio on the second stand 3 ₂ to the edge drop ratio on the third stand 3 ₃ can be suppressed to be smaller than that in a case where only the work roll 4 a of the second stand 3 ₂ is set as the tapered work roll 4 a 1. The same effect is also exhibited in a case where the second stand 3 ₂ and the third stand 3 ₃ having the two continuous tapered work rolls 4 a 1 roll the electromagnetic steel sheet with a linear load greater than 1.7 t/mm.

Accordingly, a tension applied to an edge portion on the third stand 3 ₃ is further reduced, and edge crack of the steel sheet can be more appropriately suppressed.

Therefore, in the cold rolling equipment 1 illustrated in FIG. 15 , by assuming that the steel sheet S as the material to be rolled is rolled with a linear load of 1.7 t/mm or more, each work roll 4 a of the second stand 3 ₂ is not only configured with the tapered work roll 4 a 1, but also each work roll 4 a of the third stand 3 ₃ is configured with the tapered work roll 4 a 1.

However, in a case where the linear load of the second stand 3 ₂ and the third stand 3 ₃ having the tapered work rolls 4 a 1 is set as 1.7 t/mm or more, the tapered rolling portion width WRδ of the tapered work rolls 4 a 1 of the second stand 3 ₂ and the third stand 3 ₂ is set as −50 mm to −5 mm, due to the same reason as described above in the first embodiment.

As described above, in the cold rolling equipment 1 according to the second embodiment, among the four stands of the first stands 3 ₁ to the fourth stands 3 ₄, the second stand 3 ₂ arranged in the second position from the upstream side of the steel sheet S, as the material to be rolled, in the transfer direction and the third stand 3 ₃ arranged in the third position on the downstream side of the second stand 3 ₂ include the tapered work roll 4 a 1 having the taper 4 ab formed on the end portion of the roll 4 aa having a uniform diameter. The linear load of the second stand 3 ₂ and the third stand 3 ₃ is set as 1.7 t/mm or more, and the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of each of the second stand 3 ₂ and the third stand 3 ₃ is set as −50 mm to −5 mm.

Therefore, the edge drop ratio on the first stand 3 ₁ to the fourth stand 3 ₄ is kept within the appropriate range, and the occurrence of edge crack on the steel sheet S as the material to be rolled during the cold rolling is more sufficiently suppressed, thereby suppressing the sheet breakage.

In addition, in the cold rolling equipment 1 according to the second embodiment, among the four stands of first stands 3 ₁ to the fourth stand 3 ₄, the fourth stand 3 ₄ arranged in the fourth position on the downstream side of the third stand 3 ₃ includes the flat work roll 4 a 2 having a uniform diameter.

Accordingly, the effect of promoting the edge drop is exhibited.

In addition, in the cold rolling equipment 1 according to the second embodiment, among the four stands of first stands 3 ₁ to the fourth stands 3 ₄, the first stand 3 ₁ arranged on the uppermost stream side includes the flat work roll 4 a 2, but may include the tapered work roll 4 a 1.

Accordingly, the effect of suppressing the edge drop is exhibited.

In addition, in a cold rolling method according to the second embodiment, the steel sheet S as the material to be rolled is cold-rolled by the rolling mill 2 in the cold rolling equipment 1 illustrated in FIG. 1 . In this case, the second stand 3 ₂ and the third stand 3 ₃ among the four stands of first stands 3 ₁ to fourth stands 3 ₄ include the tapered work roll 4 a 1 having the taper 4 ab formed on the end portion of the roll 4 aa having a uniform diameter. The second stand 3 ₂ and the third stand 3 ₃ roll the steel sheet S with a linear load of 1.7 t/mm or more, and by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of each of the second stand 3 ₂ and the third stand 3 ₃ as −50 mm to −5 mm.

Therefore, the edge drop ratio on the first stand 3 ₁ to the fourth stand 3 ₄ is kept within the appropriate range, and the occurrence of edge crack on the steel sheet S as the material to be rolled during the cold rolling is more sufficiently suppressed, thereby suppressing the sheet breakage.

Here, the configuration of the tapered work roll 4 a 1 is the same as that illustrated in FIG. 2 , and on the tapered work roll 4 a 1, the taper 4 ab which is tapered is formed on an end portion of the roll 4 aa having a uniform diameter in a body length direction. The work roll 4 a, which is the tapered work roll 4 a 1, is configured to be shifted in an axial direction (a roll body length direction, a sheet width direction of the steel sheet S).

When a case where the width direction end surface of the steel sheet S protrudes from the taper start end 4 ac is set as negative, and when a case where the width direction end surface of the steel sheet S is recessed than the taper start end 4 ac is set as positive, as described above, the steel sheet S is rolled by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab facing the steel sheet S as −50 mm to −5 mm. In addition, the inclination L/H of the taper 4 ab is preferably 1/800 to 1/400 as described above.

By the way, even when the tapered work roll 4 a 1 is used, if the tapered rolling portion width WRδ is greater than 0 mm on the positive side, the flat work roll 4 a 2 can be used.

Therefore, in both the cold rolling equipment 1 of the first embodiment and the second embodiment, each work roll 4 a of the second stand 3 ₂ and each work roll 4 a of the third stand 3 ₃ are set as the tapered work roll 4 a 1, and the steel sheet can also be rolled in a preferred aspect by shifting each work roll 4 a in the axial direction according to the linear load.

That is, when the linear load is 0.8 t/mm or more and less than 1.4 t/mm, the tapered work roll 4 a 1 may be used as the flat work roll 4 a 2 by shifting each work roll 4 a in the axial direction so that the tapered rolling portion width WRδ in the second stand 3 ₂ is set as ±0 mm to +50 mm and the tapered rolling portion width WRδ in the third stand 3 ₂ is greater than 0 mm.

In addition, when the linear load is 1.4 t/mm or more and less than 1.7 t/mm, the work roll may be used as the flat work roll 4 a 2 by shifting each work roll 4 a in the axial direction so that the tapered rolling portion width WRδ in the second stand 3 ₂ is set as −50 mm to −5 mm and the tapered rolling portion width WRδ in the third stand 3 ₃ is greater than 0 mm.

Furthermore, when the linear load is 1.7 t/mm or more, the work roll may be used as the tapered work roll 4 a 1 by shifting each work roll 4 a in the axial direction so that the tapered rolling portion width WRδ of each of the second stand 3 ₂ and the third stand 3 ₃ is set as −50 mm to −5 mm.

In the cold-rolled steel sheet manufacturing method according to the second embodiment, the steel sheet S is cold-rolled by the cold rolling method according to the second embodiment to manufacture the cold-rolled steel sheet.

Although the embodiments of the present invention have been described above, the present invention is not limited thereto and can be modified and improved in various ways.

In the first embodiment and the second embodiment, the rolling mill (tandem rolling mill) 2 including the four stands of the first stands 3 ₁ to the fourth stands 3 ₄ was described as an example, but the number of stands is not limited thereto, and the rolling mill 2 including five or more stands may be used. When five or more stands are provided, in a case where the number of stands including the work roll 4 a as the tapered work roll 4 a 1 is one as in the first embodiment, one stand may be selected from stands in a preceding stage, except a stand on the uppermost stream side. In addition, when the number of stands including the work roll 4 a as the tapered work roll 4 a 1 is two continuously provided as in the second embodiment, at least one stand among these may be one stand selected from the stands in a preceding stage may be used, except for a stand on the uppermost stream side.

Further, in the first embodiment, the tapered work roll 4 a 1 is applied to the second stand 3 ₂, and this is because that, in the rolling mill 2 of the first embodiment, since the breakage occurs on the third stand 3 ₃, the stand on the upstream side thereof is selected. In the second embodiment, the tapered work roll 4 a 1 is applied to the second stand 3 ₂ and the third stand 3 ₃, and this is also because the breakage occurs on the third stand 3 ₃. In the first and second embodiments, the tapered work roll 4 a 1 may be applied to other stands according to the configuration of the rolling mill (tandem rolling mill) 2 or the like.

Based on the above, in the cold rolling method, the cold rolling equipment, and the cold-rolled steel sheet manufacturing method according to the present invention, among the plurality of stands 3 ₁ to 3 ₄, the N-th stand 3 _(N) arranged in the N-th position (N is a natural number equal to or greater than 2) from the upstream side of the steel sheet S, as the material to be rolled, in the transfer direction includes the tapered work roll 4 a 1 having the taper 4 ab formed on the end portion of the roll 4 aa having a uniform diameter. Then, on the N-th stand 3 _(N), the steel sheet S as the material to be rolled is rolled with a linear load of 0.8 t/mm or more.

In addition, when the N-th stand 3 _(N) rolls the steel sheet S with a linear load of less than 1.4 t/mm, it is preferable that the steel sheet S is rolled by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of the N-th stand 3 _(N) as ±50 mm to +50 mm.

Meanwhile, when the N-th stand 3 _(N) rolls the steel sheet S with a linear load of 1.4 t/mm or more, it is preferable that the steel sheet S is rolled by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of the N-th stand 3 _(N) as −50 mm to −5 mm.

In the cold rolling method, the cold rolling equipment, and the cold-rolled steel sheet manufacturing method according to the present invention, it is preferable that, among the plurality of stands 3 ₁ to 3 ₄, the (N+1)-th stand 3 _(N+1) arranged in the (N+1)-th position on the downstream side of the N-th stand 3 _(N) includes the flat work roll 4 a 2 having a uniform diameter of the roll to roll the steel sheet S as the material to be rolled.

In addition, in the cold rolling method, the cold rolling equipment, and the cold-rolled steel sheet manufacturing method according to the present invention, among the plurality of stands 3 ₁ to 3 ₄, the N-th stand 3 _(N) arranged in the N-th position (N is a natural number equal to or greater than 2) from the upstream side of the steel sheet S, as the material to be rolled, in the transfer direction and the (N+1)-th stand 3 _(N+1) arranged in the (N+1)-th position on the downstream side of the N-th stand 3 _(N) include the tapered work roll 4 a 1 having the taper 4 ab formed on the end portion of the roll 4 aa having a uniform diameter. The N-th stand 3 _(N) and the (N+1)-th stand 3 _(N+1) rolls the steel sheet S with a linear load of 1.7 t/mm or more, and by setting the tapered rolling portion width WRδ which is a length of the taper 4 ab, facing the steel sheet S, which is formed on the tapered work roll 4 a 1 of each of the N-th stand 3 _(N) and the (N+1)-th stand 3 _(N+1) as −50 mm to −5 mm.

In the cold rolling method, the cold rolling equipment, and the cold-rolled steel sheet manufacturing method according to the present invention, it is preferable that, among the plurality of stands 3 ₁ to 3 ₄, the (N+2)-th stand 3 _(N+2) arranged in the (N+2)-th position on the downstream side of the (N+1)-th stand 3 _(N+1) includes the flat work roll 4 a 2 having a uniform diameter of the roll to roll the steel sheet S as the material to be rolled.

In addition, in the first and second embodiments, the electromagnetic steel sheet was used as the material to be rolled in the test, but the steel type of the steel sheet S is not limited to the electromagnetic steel sheet. A stainless steel sheet, a high-carbon steel sheet, an electromagnetic steel sheet, and the like are known as steel types in which edge cracks are generally likely to occur on the edge portion, and a significant effect is exhibited by using the cold rolling method, the cold rolling equipment, and the cold-rolled steel sheet manufacturing method of the present invention.

EXAMPLES

In order to verify the effect of the present invention, the cold rolling was performed using the cold rolling equipment 1 under conditions illustrated in Tables 5 and 6. In Tables 5 and 6, a rolling reduction is calculated based on a sheet thickness of the material to be rolled on an inner side of the first stand 3 ₁ and a sheet thickness of the material to be rolled on the outer side of the fourth stand 3 ₄.

TABLE 5 Edge drop Tapered work roll ratio (—) of Edge drop Presence or Material to Rolling WRδ Taper Linear load corresponding ratio (—) absence of be rolled reduction (mm) inclination Applied STD (t/mm) stand of next stand edge crack Invention Electromagnetic 90.2% −50 1/800 Second STD 1.45 −0.01 −0.02 Absent Example 1 steel sheet A Invention Electromagnetic 90.2% −30 1/800 Second STD 1.45 −0.02 −0.03 Absent Example 2 steel sheet A Invention Electromagnetic 90.2% −20 1/800 Second STD 1.45 −0.02 −0.02 Absent Example 3 steel sheet A Invention Electromagnetic 90.2% −5 1/800 Second STD 1.45 −0.04 −0.03 Absent Example 4 steel sheet A Comparative Electromagnetic 90.2% −60 1/800 Second STD 1.45 +0.02 −0.02 Present Example 1 steel sheet A Comparative Electromagnetic 90.2% +5 1/800 Second STD 1.45 −0.10 +0.05 Present Example 2 steel sheet A Comparative Electromagnetic 90.2% — — — 1.45 −0.07 +0.03 Present Example 3 steel sheet A Invention Electromagnetic 90.2% −50 1/800 Second STD 1.6 −0.02 −0.01 Absent Example 5 steel sheet B Invention Electromagnetic 90.2% −30 1/800 Second STD 1.6 −0.03 −0.04 Absent Example 6 steel sheet B Invention Electromagnetic 90.2% −20 1/800 Second STD 1.6 −0.03 −0.03 Absent Example 7 steel sheet B Invention Electromagnetic 90.2% −5 1/800 Second STD 1.6 −0.05 −0.02 Absent Example 8 steel sheet B Comparative Electromagnetic 90.2% −60 1/800 Second STD 1.6 +0.06 −0.03 Present Example 4 steel sheet B Comparative Electromagnetic 90.2% +5 1/800 Second STD 1.6 −0.11 +0.07 Present Example 5 steel sheet B Comparative Electromagnetic 90.2% — — — 1.6 −0.08 +0.02 Present Example 6 steel sheet B Invention Electromagnetic 90.8% −50 1/800 Second STD 1.65 −0.03 −0.02 Absent Example 9 steel sheet C Invention Electromagnetic 90.8% −30 1/800 Second STD 1.65 −0.04 −0.05 Absent Example 10 steel sheet C Invention Electromagnetic 90.8% −20 1/800 Second STD 1.65 −0.04 0.04 Absent Example 11 steel sheet C Invention Electromagnetic 90.8% −5 1/800 Second STD 1.65 −0.06 −0.01 Absent Example 12 steel sheet C Comparative Electromagnetic 90.8% −60 1/800 Second STD 1.65 +0.04 0.04 Present Example 7 steel sheet C Comparative Electromagnetic 90.8% +5 1/800 Second STD 1.65 −0.12 +0.07 Present Example 8 steel sheet C Comparative Electromagnetic 90.8% — — — 1.65 −0.09 +0.03 Present Example 9 steel sheet C Invention Electromagnetic 90.2% −30 1/400 Second STD 1.45 −0.01 −0.03 Absent Example 13 steel sheet A Invention Electromagnetic 93.2% −30 1/800 Second and 1.72 −0.01 −0.03 Absent Example 14 steel sheet H third STDs Invention Stainless steel 83.3% −30 1/800 Second STD 1.68 −0.03 −0.02 Absent Example 15 sheet Invention High-carbon 65.8% −30 1/800 Second STD 1.69 −0.03 −0.03 Absent Example 16 steel sheet

TABLE 6 Edge drop Tapered work roll ratio (—) of Edge drop Presence or Material to Rolling WRδ Taper Linear load corresponding ratio (—) absence of be rolled reduction (mm) inclination Applied STD (t/mm) stand of next stand edge crack Reference Electromagnetic 51.1% −5 1/800 Second STD 0.75 +0.07 −0.02 Present Example 1 steel sheet D Reference Electromagnetic 51.1% 0 1/800 Second STD 0.75 −0.01 −0.01 Absent Example 2 steel sheet D Reference Electromagnetic 51.1% 60 1/800 Second STD 0.75 −0.03 −0.02 Absent Example 3 steel sheet D Reference Electromagnetic 51.1% — — — 0.75 −0.02 −0.02 Absent Example 4 steel sheet D Invention Electromagnetic 53.6% 0 1/800 Second STD 0.85 −0.01 −0.01 Absent Example 17 steel sheet E Invention Electromagnetic 53.6% +20 1/800 Second STD 0.85 −0.01 −0.02 Absent Example 18 steel sheet E Invention Electromagnetic 53.6% +30 1/800 Second STD 0.85 −0.01 −0.02 Absent Example 19 steel sheet E Invention Electromagnetic 53.6% +50 1/800 Second STD 0.85 −0.02 −0.01 Absent Example 20 steel sheet E Comparative Electromagnetic 53.6% +60 1/800 Second STD 0.85 −0.03 +0.02 Present Example 10 steel sheet E Comparative Electromagnetic 53.6% −5 1/800 Second STD 0.85 +0.04 −0.02 Present Example 11 steel sheet E Comparative Electromagnetic 53.6% — — — 0.85 −0.03 +0.01 Present Example 12 steel sheet E Invention Electromagnetic 63.0% 0 1/800 Second STD 1.25 −0.02 −0.01 Absent Example 21 steel sheet F Invention Electromagnetic 63.0% +20 1/800 Second STD 1.25 −0.02 −0.02 Absent Example 22 steel sheet F Invention Electromagnetic 63.0% +30 1/800 Second STD 1.25 −0.03 −0.03 Absent Example 23 steel sheet F Invention Electromagnetic 63.0% +50 1/800 Second STD 1.25 −0.05 −0.02 Absent Example 24 steel sheet F Comparative Electromagnetic 63.0% +60 1/800 Second STD 1.25 −0.07 +0.03 Present Example 13 steel sheet F Comparative Electromagnetic 63.0% −5 1/800 Second STD 1.25 +0.02 −0.02 Present Example 14 steel sheet F Comparative Electromagnetic 63.0% — — — 1.25 −0.06 +0.02 Present Example 15 steel sheet F Invention Electromagnetic 70.2% 0 1/800 Second STD 1.35 −0.03 −0.02 Absent Example 25 steel sheet G Invention Electromagnetic 70.2% +20 1/800 Second STD 1.35 −0.03 −0.03 Absent Example 26 steel sheet G Invention Electromagnetic 70.2% +30 1/800 Second STD 1.35 −0.04 −0.04 Absent Example 27 steel sheet G Invention Electromagnetic 70.2% +50 1/800 Second STD 1.35 −0.06 −0.03 Absent Example 28 steel sheet G Comparative Electromagnetic 70.2% +60 1/800 Second STD 1.35 −0.08 +0.02 Present Example 16 steel sheet G Comparative Electromagnetic 70.2% −5 1/800 Second STD 1.35 +0.01 −0.03 Present Example 17 steel sheet G Comparative Electromagnetic 70.2% — — — 1.35 −0.07 +0.01 Present Example 18 steel sheet G

The rolling was performed by setting the material to be rolled as an electromagnetic steel sheet A in Invention Examples 1 to 4, setting the material to be rolled as an electromagnetic steel sheet B in Invention Examples 5 to 8, setting the material to be rolled as an electromagnetic steel sheet C in Invention Examples 9 to 12, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 1.4 t/mm or more, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −50 mm to −5 mm which is in the range. In any of Invention Examples 1 to 12, the edge drop ratio of the stand (the second stand 3 ₂) and the next stand (the third stand 3 ₃) was within the suitable range, and there were no edge cracks in each of Invention Examples 1 to 12.

In addition, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet A in Comparative Example 1, setting the material to be rolled as the electromagnetic steel sheet B in Comparative Example 4, setting the material to be rolled as the electromagnetic steel sheet C in Comparative Example 7, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 1.4 t/mm or more, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −60 mm which is beyond the range in the negative direction. In any of Comparative Examples 1, 4, and 7, the stand (the second stand 3 ₂) was edged up, and edge cracks were found in each of Comparative Examples 1, 4, and 7.

The rolling was performed by setting the material to be rolled as the electromagnetic steel sheet A in Comparative Example 2, setting the material to be rolled as the electromagnetic steel sheet B in Comparative Example 5, setting the material to be rolled as the electromagnetic steel sheet C in Comparative Example 8, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 1.4 t/mm or more, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as +5 mm which is beyond the range in the positive direction. In any of Comparative Examples 2, 5, and 8, the next stand (the third stand 3 ₃) of the stand (the second stand 3 ₂) was edged up, and edge cracks were found in each of Comparative Examples 2, 5, and 8.

In addition, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet A in Comparative Example 3, setting the material to be rolled as the electromagnetic steel sheet B in Comparative Example 6, setting the material to be rolled as the electromagnetic steel sheet C in Comparative Example 9, applying the flat work roll to all the first stand 3 ₁ to the fourth stand 3 ₄, and setting the linear load of the second stand 3 ₂ as 1.4 t/mm or more. In any of Comparative Examples 3, 6, and 9, the next stand (the third stand 3 ₃) of the stand (the second stand 3 ₂) including the flat work roll was edged up, and edge cracks were found in each of Comparative Examples 3, 6, and 9.

In addition, the rolling was performed by setting the material to be rolled as an electromagnetic steel sheet D in Reference Example 1, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 0.75 t/mm which is beyond the range, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −5 mm. In addition, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet D in Reference Example 2, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 0.75 t/mm which is beyond the range, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as 0 mm. In addition, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet D in Reference Example 3, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 0.75 t/mm which is beyond the range, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as +60 mm. Furthermore, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet D in Reference Example 4, applying the flat work roll to all the first stand 3 ₁ to the fourth stand 3 ₄, and setting the linear load of the second stand 3 ₂ as 0.75 t/mm or more which is beyond the range. In a case of Reference Example 1, due to the shifting of the second stand 3 ₂ in the negative direction under ultra-light pressure, the second stand 3 ₂ was edged up and the edge crack occurred. In a case of Reference Examples 2 to 4, the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ was changed to ±0 mm, +60 mm, and the work roll was changed to flat work roll. The edge drop ratio of the stand (the second stand 3 ₂) and the next stand (the third stand 3 ₃) was within the suitable range under ultra-light pressure, and in each of the Reference Examples 2 to 4, there were no edge cracks and the effect of setting the second stand 3 ₂ to the tapered work roll 4 a 1 was not exhibited.

In addition, the rolling was performed by setting the material to be rolled as an electromagnetic steel sheet E in Invention Examples 17 to 20, setting the material to be rolled as an electromagnetic steel sheet F in Invention Examples 21 to 24, setting the material to be rolled as an electromagnetic steel sheet G in Invention Examples 25 to 28, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 0.8 t/mm or more and less than 1.4 t/mm which is in the range, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as ±0 mm to +50 mm which is in the range. In any of Invention Examples 17 to 25, the edge drop ratio of the stand (the second stand 3 ₂) and the next stand (the third stand 3 ₃) was within the suitable range, and there were no edge cracks in each of Invention Examples 1 to 12.

The rolling was performed by setting the material to be rolled as the electromagnetic steel sheet E in Comparative Example 10, setting the material to be rolled as the electromagnetic steel sheet F in Comparative Example 13, setting the material to be rolled as the electromagnetic steel sheet G in Comparative Example 16, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 0.8 t/mm or more and less than 1.4 t/mm, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as +60 mm which is beyond the range in the positive direction. In any of Comparative Examples 10, 13, and 16, the next stand (the third stand 3 ₃) of the stand (the second stand 3 ₂) was edged up, and edge cracks were found in each of Comparative Examples 12, 15, and 18.

In addition, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet E in Comparative Example 11, setting the material to be rolled as the electromagnetic steel sheet F in Comparative Example 14, setting the material to be rolled as the electromagnetic steel sheet G in Comparative Example 17, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 0.8 t/mm or more and less than 1.4 t/mm, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −5 mm which is beyond the range in the negative direction. In any of Comparative Examples 11, 14, and 17, the stand (the second stand 3 ₂) was edged up, and edge cracks were found in each of Comparative Examples 11, 14, and 17.

In addition, the rolling was performed by setting the material to be rolled as the electromagnetic steel sheet E in Comparative Example 12, setting the material to be rolled as the electromagnetic steel sheet F in Comparative Example 15, setting the material to be rolled as the electromagnetic steel sheet G in Comparative Example 18, applying the flat work roll to all the first stand 3 ₁ to the fourth stand 3 ₄, and setting the linear load of the second stand 3 ₂ as 0.8 t/mm or more and less than 1.4 t/mm. In any of Comparative Examples 14, 17, and 20, the next stand (the third stand 3 ₃) of the stand (the second stand 3 ₂) including the flat work roll was edged up, and edge cracks were found in each of Comparative Examples 12, 15, and 18.

In addition, the rolling was performed by setting the material to be rolled as an electromagnetic steel sheet A in Invention Example 13, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 1.45 t/mm, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −30 mm which is in the range. The inclination L/H of the taper 4 ab of the tapered work roll 4 a 1 was set to 1/400, which is in the preferred range. In Invention Example 13, the edge drop ratio of the stand (the second stand 3 ₂) and the next stand (the third stand 3 ₃) was in the suitable range and there were no edge cracks.

In addition, the rolling was performed by setting the material to be rolled as an electromagnetic steel sheet H in Invention Example 14, applying the tapered work roll 4 a 1 to the second stand 3 ₂ and the third stand 3 ₃, setting the linear load of the second stand 3 ₂ and the third stand 3 ₃ as 1.72 t/mm, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of each of the second stand 3 ₂ and the third stand 3 ₃ as −30 mm which is in the range. In Invention Example 14, the edge drop ratio of the stand (the second stand 3 ₂) and the next stand (the third stand 3 ₃) was in the suitable range and there were no edge cracks.

In addition, the rolling was performed by setting the material to be rolled as a stainless steel sheet in Invention Example 15, setting the material to be rolled as a high-carbon steel sheet in Invention Example 16, applying the tapered work roll 4 a 1 to the second stand 3 ₂, setting the linear load of the second stand 3 ₂ as 1.4 t/mm or more, and setting the tapered rolling portion width WRδ of the tapered work roll 4 a 1 of the second stand 3 ₂ as −30 mm which is in the range. In any of Invention Examples 15 and 16, the edge drop ratio of the stand (the second stand 3 ₂) and the next stand (the third stand 3 ₃) was within the suitable range, and there were no edge cracks in each of Invention Examples 15 and 16.

REFERENCE SIGNS LIST

-   -   1 cold rolling equipment     -   2 rolling mill     -   3 ₁ first stand     -   3 ₂ second stand     -   3 ₃ third stand     -   3 ₄ fourth stand     -   3 _(N) N-th stand     -   3 _(N+1) (N+1)-th stand     -   3 _(N+2) (N+2)-th stand     -   4 a work roll     -   4 a 1 tapered work roll     -   4 a 2 flat work roll     -   4 aa roll     -   4 ab taper     -   4 ac taper start end     -   4 b back-up roll     -   4 c intermediate roll     -   S steel sheet (material to be rolled) 

1. A cold rolling method for cold-rolling a material to be rolled by a rolling mill including a plurality of stands, wherein an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and the N-th stand rolls the material to be rolled with a linear load of 0.8 t/mm or more.
 2. The cold rolling method according to claim 1, wherein, in a case where the N-th stand rolls the material to be rolled with a linear load of less than 1.4 t/mm, the material to be rolled is rolled by setting a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of the N-th stand as ±0 mm to +50 mm.
 3. The cold rolling method according to claim 1, wherein, in a case where the N-th stand rolls the material to be rolled with a linear load of 1.4 t/mm or more, the material to be rolled is rolled by setting a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of the N-th stand as −50 mm to −5 mm.
 4. The cold rolling method according to claim 1, wherein, among the plurality of stands, an (N+1)-th stand arranged in an (N+1)-th position on a downstream side of the N-th stand rolls the material to be rolled with a flat work roll having a uniform diameter of a roll.
 5. A cold rolling method for cold-rolling a material to be rolled by a rolling mill including a plurality of stands, wherein an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction and an (N+1)-th stand arranged in an (N+1)-th position on a downstream side of the N-th stand among the plurality of stands, include a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and each of the N-th stand and the (N+1)-th stand rolls the material to be rolled with a linear load of 1.7 t/mm or more and a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of each of the N-th stand and the (N+1)-th stand is set as −50 mm to −5 mm.
 6. The cold rolling method according to claim 5, wherein, among the plurality of stands, an (N+2)-th stand arranged in an (N+2)-th position on a downstream side of the (N+1)-th stand rolls the material to be rolled with a flat work roll having a uniform diameter of a roll.
 7. The cold rolling method according to claim 1, wherein, among the plurality of stands, a stand arranged on an uppermost stream side of the material to be rolled in the transfer direction rolls the material to be rolled with a flat work roll having a uniform diameter of a roll.
 8. Cold rolling equipment including a rolling mill having a plurality of stands for cold-rolling a material to be rolled, wherein an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and a linear load of the N-th stand is set as 0.8 t/mm or more.
 9. The cold rolling equipment according to claim 8, wherein, in a case where a linear load of the N-th stand is set as less than 1.4 t/mm, a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of the N-th stand is set as ±0 mm to +50 mm.
 10. The cold rolling equipment according to claim 8, wherein, in a case where a linear load of the N-th stand is set as 1.4 t/mm or more, a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of the N-th stand is set as −50 mm to −5 mm.
 11. The cold rolling equipment according to claim 8, wherein, among the plurality of stands, an (N+1)-th stand arranged in an (N+1)-th position on a downstream side of the N-th stand includes a flat work roll having a uniform diameter of a roll.
 12. Cold rolling equipment including a rolling mill having a plurality of stands for rolling a material to be rolled, wherein each of an N-th stand (N is a natural number equal to or greater than 2) arranged in an N-th position from an upstream side of the material to be rolled in a transfer direction and an (N+1)-th stand arranged in an (N+1)-th position on a downstream side of the N-th stand among the plurality of stands, includes a tapered work roll having a taper formed on an end portion of a roll having a uniform diameter, and a linear load of each of the N-th stand and the (N+1)-th stand is set as 1.7 t/mm or more, and a tapered rolling portion width WRδ which is a length, which faces the material to be rolled, of the taper formed on the tapered work roll of each of the N-th stand and the (N+1)-th stand is set as −50 mm to −5 mm.
 13. The cold rolling equipment according to claim 12, wherein, among the plurality of stands, an (N+2)-th stand arranged in an (N+2)-th position on a downstream side of the (N+1)-th stand includes a flat work roll having a uniform diameter of a roll.
 14. The cold rolling equipment according to claim 8, wherein, among the plurality of stands, a stand arranged on an uppermost stream side of the material to be rolled in the transfer direction includes a flat work roll having a uniform diameter of a roll.
 15. A cold-rolled steel sheet manufacturing method comprising: cold-rolling a steel sheet to manufacture a cold-rolled steel sheet by the cold rolling method according to claim
 1. 